The present invention relates to an electronic component formed by mounting an ultra high frequency device in a compact, hollow package and, more particularly, to the shape of a lead member into which the solder is difficult to penetrate.
Examples of the ultra high frequency device are a transistor, an IC, an optical element, a surface acoustic wave element, and a resonator. These devices are used in commercial communication equipments and satellites, and must have a high reliability and a long service life even if they may be somewhat expensive. Accordingly, such a device is conventionally incorporated in a ceramic hollow package.
In recent years, many ultra high frequency devices are used in a home-use equipment, e.g., the receiver of satellite broadcast or a portable telephone as well. Although a device used in the home-use equipment need not have such a high reliability and a long service like those in the commercial communication equipment, it must be manufactured at a low cost, which is the major issue. As the equipment is made compact, as in the case of the portable telephone, the outer shape of its ultra high frequency device must accordingly be made more compact than ever before.
Recently, a device, which has conventionally been incorporated in a ceramic hollow package, tends to be incorporated in a resin-molded package for the purpose of cost reduction. When, however, the ultra high frequency chip is entirely sealed with a resin, the parasitic capacitance increase because the resin has a high dielectric constant, leading to a large high-frequency loss.
FIGS. 7, 8A, and 8B show an electronic component formed by mounting an ultra high frequency device chip (to be merely referred to as a chip hereinafter) in a conventional hollow package. FIG. 7 shows a state wherein the cap is removed, FIG. 8A shows a state wherein the cap is placed, and FIG. 8B shows a section of FIG. 7.
Referring to FIG. 7, an electronic component 20 is constituted by an octagonal base 2 on which lead members 4 and 5 made of conductive strip pieces and arranged crosswise are sealed with a resin, a cap 8 (FIG. 8A) adhered to the base 2, a chip 3 mounted on an island 5a, and bonding wires 6 for electrically connecting the chip 3 and lead members 4 and 5 to each other. The lead members 4 and 5 and the island 5a constitute a lead frame. The cap 8 is adhered to the base 2, thereby constituting a hollow package in which a space is formed between the chip 3 and cap 8.
The base 2 is integrally molded with the lead members 4 and 5 by loading the lead members 4 and 5, before the chip is mounted on them, in a sealing mold, and injecting an epoxy resin in the sealing mold and solidifying the epoxy resin. The main body of the base 2 forms a bottomed cylinder consisting of a substrate portion 2a and an annular bank portion 2b. The lead members 4 and 5 are fixed to the chip support surface of the substrate portion 2a. The bank portion 2b is annularly formed on the periphery of the substrate portion 2a to surround the chip support surface like a frame. The bank portion 2b functions such that, even if a mechanical stress is applied to the base 2, the resin will not peel off from the lead members 4 and 5.
The lead members 4 and 5 are fixed between the substrate portion 2a and bank portion 2b, and their portions where they are in contact with the bank portion 2b are called bank portion leads 4d and 5d (indicated by crosshatched lines in FIG. 7). One surface of the island 5a constituted by the inner lead of the lead member 5, and one surface of an inner lead 4a of each lead member 4 are exposed to the inner bottom surface of the base 2, i.e., to the chip support surface of the substrate portion 2a, and outer leads 4b and 5b extend from the base 2 outwardly in four directions.
The cap 8 is molded by injecting an epoxy resin in a sealing mold and solidifying the epoxy resin. This cap 8 has a recessed portion 8a to oppose the base 2, and the periphery of the recessed portion 8a is adhered to the upper surface of the bank portion 2b with an adhesive 9, as shown in FIG. 8A.
The characteristic feature of this high frequency hollow package resides in that its outer shape is very small. For example, an outer diameter D of the package is 2 mm, a width W of the lead members 4 and 5 is 0.5 mm, and the width of the bank portion 2b is 0.35 mm. Since the outer shape of the hollow package is very small in this manner, the shearing strength of the bank portion 2b must be reinforced. For this purpose, anchor holes 4c and 5c are formed in the bank portion leads 4d and 5d , respectively, and are filled with a resin, thereby connecting the substrate portion 2a and bank portion 2b to each other. The diameter of the anchor holes 4c and 5c is 0.2 mm.
When the electronic component 20 with the hollow package structure constituted in this manner is to be mounted on a circuit board, the electronic component 20 is dipped in a solder bath so that the outer leads 4b and 5b are electrically connected to the conductor pattern (not shown) of the circuit board. At this time, the solder or a flux 10 may undesirably penetrate into the hollow package to fuse the bonding wires 6, and the electronic component 20 may accordingly malfunction. Alternatively, the solder or flux 10 may undesirably attach to the surface of the chip 3 to degrade the electrical characteristics of the chip 3, causing defective components. It was found that what allowed penetration of the solder or flux 10 was a gap 7 formed between the end of each of the lead members 4 and and the resin of the base 2, as shown in FIG. 8B.
The reason why the gap 7 is formed is as follows. When the lead members 4 and 5 are formed by etching, the ends of the lead members 4 and 5 that are not masked are etched excessively. When the lead members 4 and 5 are formed by press cutting, they are cut partly differently to form recessed portions 4g at the ends of the lead members 4 and 5. A depth .alpha. of the recessed portions 4g is as very small as about 10 .mu.m to 30 .mu.m. Since the resin particles of the epoxy resin that integrally molds the lead members 4 and 5 with the base 2 vary between 10 .mu.m and 100 .mu.m, if large particles happen to be sealed in the recessed portions 4g, the epoxy resin cannot completely fill the recessed portions 4g, but the gap 7 is formed.
In a package having a large outer shape, as in the conventional case, since the lead member portions covered with the resin are long, the probability of the resin with a small particle size to fill the gap increases, and the problem of solder penetration does not substantially occur. In contrast to this, an electronic component with a hollow package structure has a very small outer shape, and the width of its resin partition, i.e., the bank portion 2b, is as small as 0.35 mm. It is estimated that, in such an electronic component, the probability of the resin with a small particle size to fill the gap decreases, and that the gap 7 is formed with a high probability.
A technique that prevents the solder or flux 10 from penetrating into the package through the gap 7 between the lead members 4 and 5 and the sealing resin is not conventionally found, since there is substantially no probability of solder penetration. As a similar technical concept, a technique that prevents penetration of moisture is present, as shown in FIGS. 9 to 12. A case in which such a technical concept is applied to an electronic component having a hollow package structure will be described hereinafter.
FIGS. 9A and 9B show the main part of a semiconductor device as the first conventional example disclosed in Japanese Patent Laid-Open No. 2-14555. Referring to FIGS. 9A and 9B, waved portions 11 are formed on the two surfaces of a portion corresponding to a bank portion lead 4d of a lead member 4. With this arrangement, the adhesion between the bank portion lead 4d and the sealing resin is improved, thereby improving the moisture penetration resistance.
FIG. 10 shows the main part of an electronic component as the second conventional example disclosed in Japanese Patent Laid-Open No. 55-99755. Referring to FIG. 10, projections 12a and 12b are formed at two ends of a portion corresponding to a bank portion lead 4d of a lead member 4, in order to prevent the lead member 4 from being removed from the sealing resin. However, this reference has no description concerning penetration of the solder or moisture into the package.
FIG. 11 shows the main part of an electronic component as the third conventional example disclosed in Japanese Patent Laid-Open No. 2-78262. Referring to FIG. 11, a crank portion 13 is formed at a portion corresponding to a bank portion lead 4d of a lead member 4, so that the moisture resistance is improved.
FIG. 12 shows the main part of a semiconductor device as the fourth conventional example disclosed in Japanese Patent Laid-Open No. 5-226548. Referring to FIG. 12, an annular portion 14 having an anchor hole 4c is formed at a portion corresponding to a bank portion lead 4d of a lead member 4. Furthermore, the width of the annular portion 14 is made larger than that of the lead member 4, and the two edges of the annular portion 14 are made to project in an arcuated manner. With this arrangement, the mechanical impact applied to the lead member 4 during assembly is dispersedly absorbed, and the moisture penetrating into the package through the lead member 4 is blocked.
In the first conventional example described above, although penetration of the solder into the package from the two surfaces of the lead member 4 can be prevented, solder penetration from the two ends of the lead member 4 cannot be prevented. When this arrangement is applied to a compact hollow package, the waved portions 11 are formed to extend to a portion near the outer lead 4b. Accordingly, when the outer lead is bent, the stress is concentrated on the recessed portions, and the proximal portion of the outer lead can be easily broken.
In the second conventional example, the proportion of the area occupied by the bank portion lead 4d, including the projections 12a and 12b, to the area of the portion that connects a bank portion 2b and a substrate portion 2a becomes large, and the resin of the bank portion 2b corresponding to the bank portion lead 4d tends to peel off. As a result, if a foreign matter abuts against the cap or the outer lead is tried to be bent, a crack may be formed in the bank portion 2b or the bank portion 2b may be fractured. In this manner, when the projections 12a and 12b are formed on the bank portion lead 4d, the shearing strength of the bank portion 2b decreases.
In the third conventional example, a lead width .beta. of the crank portion 13 formed in the bank portion lead 4d decreases, and the tensile strength of the lead member 4 decreases extremely. The reason for this is as follows. As described above, a high frequency hollow package has a very small outer shape and the width of its bank portion 2b is as small as 0.35 mm. A crank-like lead cannot be formed unless the lead width D of the crank portion 13 is set to be equal to or smaller than 0.05 mm. Also, since the lead member 4 becomes long more than necessary, its inductance increases to decrease the high frequency characteristics.
In the fourth conventional example, if the diameter of the anchor hole 4c of the annular portion 14 remains to be 0.2 mm, the effect of increasing the length of the moisture penetration path cannot be obtained unless a width W of the lead member 4 is set to be equal to or smaller than 0.2 mm. Then, the tensile strength of the lead member 4 decreases, and the high frequency characteristics degrade. On the contrary, if the width W of the lead member 4 remains to be 0.5 mm, the length of moisture penetration path cannot be increased unless the diameter of the anchor hole 4c is increased to about 0.5 mm and unless the diameter of the annular portion 14 is increased to about 1 mm. For this reason, the proportion of the area occupied by the annular portion 14 increases, and the shearing strength of the bank portion 2b decreases accordingly. As the width of the bank portion 2b is as small as 0.35 mm, the anchor hole 4c and bank portion lead 4d may extend beyond the bank portion 2b. In this case, the length of the moisture penetration path cannot be increased, resulting in a contradiction. Namely, the fourth conventional example can be applied to a large package but cannot be applied to a small package.
In this manner, even if the conventional technique for improving the moisture penetration resistance and that for improving the mechanical strength are to be applied to an electronic component having a hollow package structure, due to the extreme small size of the outer shape of the package, the mechanical strength of the hollow package decreases, and the high frequency characteristics of the electronic component degrade.